Next generation sequencing has revealed activating myeloid differentiation primary response 88 (MYD88) mutations in several B-cell malignancies including Waldenstrom's Macroglobulinemia (IgM secreting lymphoplasmacytic lymphoma), non-IgM secreting lymphoplasmacytic lymphoma, ABC subtype of diffuse large B-cell lymphoma, primary central nervous system (CNS) lymphoma, immune privileged lymphomas that include testicular lymphoma, marginal zone lymphoma, and chronic lymphocytic leukemia. Particularly striking has been the expression of MYD88 mutations in Waldenstrom Macroglobulinemia (WM), wherein 95-97% of patients express MYD88L265P, and more rarely non-L265P MYD88 mutations. WM is considered to correspond to lymphoplasmacytic lymphoma (LPL) as defined by the World Health Organization classification system. Up to 30% of patients with Activated B-Cell (ABC) Subtype of Diffuse Large B-cell lymphoma (ABC DLBCL) also express activating MYD88 mutations, including MYD88L265P. Mutations in MYD88 promote Myddosome self-assembly and can trigger NF-kB signaling in the absence of Toll (TLR) or IL1 (IL1R) receptor signaling through IL1 Receptor Associated Kinases (IRAK4/IRAK1) or Bruton's Tyrosine Kinase (BTK).
Ibrutinib is an inhibitor of BTK that is highly active in WM, producing responses in 91% of previously treated patients. In WM patients, both major and overall responses to ibrutinib are higher in patients with MYD88 mutations. Ibrutinib also shows activity in previously treated patients with ABC DLBCL, particularly among patients with MYD88 mutations. Ibrutinib is also active in other B-cell malignancies including Chronic Lymphocytic Leukemia (CLL) and Mantle Cell Lymphoma (MCL). Suppression of tonic B-cell receptor (BCR) activity mediated by BTK has been implicated as the mechanism underlying ibrutinib activity in non-WM B-cell diseases.